The same applies if you come from the Cologne-Bonn-Airport (via A59), then as above, or from the east of Cologne (via A59 or via A3, and A560), then as above. Direction Bonn is always indicated on these motorways.
If you're coming from the Cologne direction, take the A555 until the "Autobahnkreuz Bonn-Nord" and then the A565 towards Koblenz.
From the Koblenz direction, take the A61 to the "Autobahnkreuz Meckenheim" and switch to the A565 towards Bonn.
You leave the motorway at the Bonn-Poppelsdorf exit (as the Institute is in Poppelsdorf). This exit makes you automatically pass under the motorway from which you came (resp. leads you directly to the two track road), and you arrive at a two track road; this road first passes under a block.
At the first traffic light go right, at the next one right again, and at the third one right again (so as to acomplish a 270 degree rotation). You're almost there.
The fourth street on the left (after Kekule-Str., Karlrobert-Kreiten-Str., Katzenburgweg) is the Nussallee, on which the Institute of Physics is. Just follow this street for some 200 meters, the Institute is on the right and has the number 12.
The Institute of Physics
Download the Official directions folder of Bonn University (available in German only).